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cmd/compile: recognize integer ranges in switch statements

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Consider a switch statement like:

switch x {
case 1:
  // ...
case 2, 3, 4, 5, 6:
  // ...
case 5:
  // ...
}

Prior to this CL, the generated code treated
2, 3, 4, 5, and 6 independently in a binary search.
With this CL, the generated code checks whether
2 <= x && x <= 6.
walkinrange then optimizes that range check
into a single unsigned comparison.

Experiments suggest that the best min range size
is 2, using binary size as a proxy for optimization.

Binary sizes before/after this CL:

cmd/compile: 14209728 / 14165360
cmd/go:       9543100 /  9539004

Change-Id: If2f7fb97ca80468fa70351ef540866200c4c996c
Reviewed-on: https://go-review.googlesource.com/26770
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
This commit is contained in:
Josh Bleecher Snyder 2016-06-17 16:27:23 -07:00
parent 5c3edc46a6
commit 8c85e23087
3 changed files with 141 additions and 22 deletions
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2
src/cmd/compile/internal/gc/syntax.go
View file

@ -436,7 +436,7 @@ const (
// statements
OBLOCK // { List } (block of code)
OBREAK // break
OCASE // case Left: Nbody (select case after processing; Left==nil means default)
OCASE // case Left or List[0]..List[1]: Nbody (select case after processing; Left==nil and List==nil means default)
OXCASE // case List: Nbody (select case before processing; List==nil means default)
OCONTINUE // continue
ODEFER // defer Left (Left must be call)